Preeclampsia is a major complication of pregnancy characterized by hypertension (HTN) and proteinuria; however, the underlying mechanisms are unclear. Reduction in uterine perfusion pressure (RUPP) in late pregnant rats is associated with HTN, increased vasoconstriction and reduced relaxation. In search for the mechanisms linking RUPP to HTN, we have found that elevation of plasma levels of tumor necrosis factor- (TNF) in late-Preg rats is associated with decreased vascular relaxation, and increased vasoconstriction and blood pressure (BP). Also, RUPP is associated with increased plasma levels of soluble fms-like tyrosine kinase-1 (sFlt-1), and chronic elevation of sFlt-1 in pregnant rats causes increases in BP. However, the central vascular targets involved are unclear. Endothelin-1 (ET-1) is a major vasoconstrictor associated with some forms of HTN including HTN-Preg. ET-1 activates endothelin A receptor (ETAR) in vascular smooth muscle to induce vasoconstriction, and ETBR in the endothelium to induce the release of vasodilator substances. Although the role of ETAR in vasoconstriction and HTN has been studied extensively, the role of vascular ETBR, particularly during pregnancy, is less clear. Our data suggest that ET-1 induced vasoconstriction is reduced in mesenteric microvessels of Norm-Preg compared to virgin rats and enhanced in RUPP rats. Also, the ETBR agonists IRL-1620 and sarafotoxin c (S6c) cause greater dilation in microvessels of Norm-Preg than virgin and RUPP rats, suggesting that ETBR-mediated relaxation pathways are upregulated during Norm-Preg and reduced in HTN-Preg. The overall objective of this proposal is to test the central hypothesis that ETBR is an important regulator of microvascular function and BP during pregnancy. During Norm-Preg, upregulation of endothelial ETBR leads to enhanced microvascular relaxation, and reduced vasoconstriction and BP. Decreased expression/activity of ETBR play a role in the endothelial cell dysfunction and vasoconstriction in HTN-Preg; and consequently, increasing the activity of ETBR should promote vasodilation and decrease BP in HTN-Preg. Studies will be performed on virgin, Norm-Preg, RUPP, TNF- and sFlt-1 infused pregnant Sprague-Dawley rats and ETBR-deficient mice. In addition to measuring BP, ex vivo functional studies and in vitro molecular and mechanistic studies will be performed on pressurized mesenteric and uterine microvessels. The specific aims are to test whether: 1) During Norm-Preg, upregulation of vascular ETBR leads to enhanced vascular relaxation, blunting of vasoconstriction, and reduction in BP. 2) Decreased expression/activity of ETBR play a role in the endothelial dysfunction and enhanced vasoconstriction associated with HTN-Preg. 3) Enhancing ETBR activity is a central target to promote vasodilation and reduce BP in HTN-Preg. These studies should help define the role of ETBR in enhancing vascular relaxation and reducing BP during Norm-Preg, provide a better understanding of the changes in the vascular ETBR in the pathogenesis of HTN-Preg, and highlight the potential usefulness of increasing the activity of ETBR in the management of preeclampsia.